The long-term objective of this research is increased understanding of the pathophysiology of neonatal hyperbilirubinemia so that this common health problem may be alleviated. The specific aim of this proposal is to test three current hypotheses which relate breast-milk composition to neonatal jaundice. These hypotheses suggest: (1) Elevated breast-milk lipase activity is present in certain milks. The lipase-catalyzed hydrolysis of milk triglycerides to free fatty acids is, therefore, increased and high concentrations of fatty acids are absorbed by the neonate that inhibit hepatic bilirubin glucuronyltransferase (BGT). Decreased BGT activity impairs bilirubin conjugation and excretion resulting in hyperbilirubinemia. (2) An unknown inhibitor(s) present in certain human milk samples similarly inhibits either BGT or another step in the hepatic clearance of bilirubin. (3) Alternatively, human milk may not interfere with hepatic bilirubin excretion but, rather, promote increased intestinal absorption of bilirubin, thus decreasing clearance. This could occur due to Beta-glucuronidase, an enzyme in human milk and neonatal intestinal tissue and bacteria which cleaves glucuronic acid from bilirubin glucuronides producing unconjugated bilirubin which is more easily absorbed from the gut. A prospective protocol involving 100 breast- and 50 formula-fed infants and their mothers will address these hypotheses by examining breast-milk, formula, serum and stool samples. Analyses will include lipase (lipoprotein lipase and bile salt stimulated lipase), free fatty acids, BGT inhibition, Beta-glucuronidase, total and direct bilirubin, total lipid, protein, albumin, bile salts and stool bile pigments. A new assay will be developed to test for inhibition of all steps of bilirubin metabolism using the intact hepatocyte in order to complement more limited microsomal assays. In 15 neonates from each feeding group quantitative stool cultures will be obtained along with milk analyses of immunologic components (IgG, A, M, SIgA, E.coli antibodies, lymphocyte subpopulations, lactoferrin and lysozyme). This will allow testing of the related hypotheses that immunologic factors in milk foster the development of the characteristic gut flora with high Beta-glucuronidase activity seen in breast-fed infants. Statistical analysis will compare the above data as a function of feeding composition and bilirubin level.